After Iraqi officials declared Fallujah fully liberated, the U.S. envoy to the coalition against Islamic State (IS) militants told lawmakers on Tuesday "planning is now underway" for a military campaign to free Mosul from the terrorist group.

"We will not put a timeline on the Mosul operation. But with momentum now on our side, it is safer to say that ISIL's days in Mosul - where it proclaimed its phony caliphate to the world - are numbered," Special Presidential Envoy of the Global Coalition to Counter ISIL Brett McGurk said in his written testimony to the Senate Committee on Foreign Relations hearing, using an another acronym for Islamic State.

After weeks of ferocious fighting, Iraqi forces ousted Islamic State extremists from Fallujah over the weekend with the support of U.S.-led coalition airstrikes.

McGurk told senators: "Mosul will be a significant military challenge but also a political, diplomatic and humanitarian challenge. The planning is now underway."

A road sign points the way to Mosul, June 6, 2016. (S. Behn/VOA)

Mosul, once home to millions of people, fell into control by Islamic State militants about two years ago. Iraqi leaders have vowed to liberate Mosul this year.

The U.S. is working through the coalition to ensure resources are ready to support Mosul’s displaced population and lay the groundwork for their return and stabilization after Islamic State control.

The U. S. said morale inside the Islamic extremist group is plunging while coalition forces against it are gaining momentum.

The State Department called the liberation of Fallujah a “major step towards the total defeat of Daesh in Iraq.” Daesh is the Arabic acronym for Islamic State terrorists.

The U.S. will host a pledging conference in Washington on July 20 to raise support for humanitarian and de-mining needs in Iraq. Participants will include representatives from Canada, Japan, Germany and other countries.